Proteins are tenaciously bound to messenger ribonucleic acid (mRNA) in the cytoplasm of mammalian cells forming messenger ribonucleoprotein (mRNP) complexes. These mRNP particles occur unattached to ribosomes in cytoplasmic pools and engaged in translation in polyribosomes. Polysomal mRNP can be released as free particles by treatment with physical or chemical disruptive agents or as a consequence of translational level inhibition of protein synthesis. Tumor cells reversibly accumulate cytoplasmic pools of inactive mRMP when protein synthesis is repressed at an initiation step by amino acid deprivation. The objective of the proposed study is to identify and characterize the protein components of mRNP from Ehrlich ascites cells and determine their possible role in translational level control of protein synthesis. Oligo (dT)-cellulose affinity chromatography and CsSO4 equilibrium density gradient centrifugation will primarily be used to isolate and characterize mRNP particles. Specific experimentation is designed to 1) identify and compare, by SDS-polyacrylamide gel electrophoresis, proteins associated with mRNA from cells actively engaged in protein synthesis and those which are repressed in translation by amino acid deprivation, high temperature shock, and arrest in mitosis; 2) classify mRNP proteins according to their association with the non-poly(A) or the poly(A) region of mRNA by specific ribonuclease elution of proteins from nRNP immobilized on poly(U)-sepharose columns, and 3) determine the arrangement of proteins on mRNA by limited fragmentation of mRNP.